1. Field of The Invention
The present invention relates to an apparatus for controlling an electric vehicle, and more particularly to an control apparatus for an electric vehicle driven by chopper-controlled electric motors.
2. Description of The Related Art
In an electric vehicle having chopper-controlled driving or traction motors, the control of both powering and braking operations is carried out by a common chopper. A control apparatus for such an electric vehicle is disclosed in Hitachi Review Vol. 33 (1984), No. 1, pp. 11 to 13. According to the apparatus disclosed therein, there is provided a line switch for changing over a main circuit of the electric vehicle from the circuit arrangement for powering to that for braking, and vice versa. For example, when a powering command occurs, the line switch is closed to form the circuit arrangement for powering, and when a braking command occurs, it is opened to change over the main circuit to the arrangement for braking.
By the way, when an electric vehicle is subjected to a so-called automatic train operation (ATO) control, the vehicle is controlled so as to make its speed follow a predetermined speed pattern, in which a running mode of the vehicle is often changed repeatedly between the powering and the braking operations. The line switch must operate every time the running mode changes, i.e., whenever the command thereof is changed.
The line switch must be operated under the condition that the current of the main circuit is zero. On the other hand, because of inductance of the main circuit, the current flowing therethrough does not become zero at once when the conduction ratio .gamma. of the chopper is made zero in response to removal of the powering command. Therefore, an instruction of opening the line switch, which is generated when the main circuit current is detected to be zero, is delayed. In addition, as is well known, the line switch itself has a time delay in its operation. As a result, the actual opening of the line switch is considerably delayed from the disappearance of the powering command.
Therefore, even though the braking command occurs just after the disappearance of the powering command, the construction of the circuit arrangement for braking and therefore the start of braking is delayed. In the ATO-controlled electric vehicle as described above, the time delay in the start of braking becomes a serious problem, because it remarkably damages the followability of the vehicle speed to the predetermined speed pattern.